This invention relates to the conversion of chemical energy to electrical energy, and more particularly to a new and improved lithium-iodine cell.
One area of use of the present invention is in providing electrical power safely to inaccessable devices in the human environment, for example to an implanted cardiac pacemaker, although the principles of the invention can be variously applied. Several types of batteries for implantable cardiac pacemakers have been proposed but heretofore all have certain limitations. Recently, a lithium-iodine cell has been proposed which advantageously has an open circuit voltage about twice that of the mercury cell, does not generate gas during the operation, and has a non-corrosive electrolyte.
A cell has been proposed including a lithium anode and a cathode comprising a charge transfer complex of an organic donor component and iodine. A typical cathode of this type of cell includes polyvinyl pyridine complexed with iodine and having excess iodine reacted in. The cathode is rendered conductive by the charge transfer complex and, at the same time, it is a diffusion source due to the excess iodine. The excess iodine insures a continuing minimal level of iodine in the complex to maintain good electrical conductivity. Having a large source of excess iodine in the cathode complex, however, can lead to many problems.
In a recently developed method for making such a cell, the organic-iodine complex is introduced to a casing, which includes the remaining cell components in the form of a pourable, tar-like or viscous substance which may be heated to an elevated temperature. When the cell is poured, the molten complex may immediately cool upon striking the lithium anode and pure non-conductive iodine may crystallize on the anode surface, thereby removing that part of the anode surface from normal operation. Also, excess iodine in the complex may adversely affect the viscosity of the cathode material thereby resulting in unwanted iodine seepage, pouring difficulties, and related problems. Furthermore, in a cathode including an organic donor component complexed with iodine and having excess iodine reacted in, only about half of the total iodine is available for electrical energy output. The remainder of the iodine appears to become permanently bound into the organic donor component matrix and thus becomes unavailable for electrical output. Thus the extent to which iodine is unavailable for electrical output results in a decrease in the energy density of the cell from the level which it could be if that iodine were available.